Various sensors have been developed to detect chemical elements and/or chemical compounds in a gas stream. For example, one Zeolite-based sensor exhibits a complex impedance that is dependent on the concentration of ammonia (NH3) in a gas stream presented to the sensor. In one particular implementation, a Zeolite-based sensor has been positioned within an exhaust gas stream of a diesel engine to provide feedback, as to the concentration of ammonia in the exhaust gas stream, to a control unit. Based upon the concentration of the ammonia in the exhaust gas stream, the control unit may cause reduction of the injection of urea, which acts to reduce nitrogen oxide (NOx) emission levels from the diesel engine, into the exhaust gas stream.
In many applications, in determining an impedance of a sensor it is undesirable to bias the sensor with a signal that has a direct current (IDC) component, as the DC component may cause ion migration or other chemical reactions in the sensor. Ion migration in a sensor may alter the impedance or other characteristics of the sensor, thereby providing an incorrect indication of the level of a gas within a gas stream. A traditional approach for determining the impedance of a sensor has utilized a system that has sourced a sinewave voltage excitation to an input of the sensor and has observed the resulting sinusoidal current. In general, such systems have captured both the amplitude and phase relationship of the sensor voltage and the sensor current, such that both a real and imaginary part of a sensor impedance could be determined.
In general, Zeolite-based sensing elements cannot be biased with any DC voltage component, as this causes ion migration in the Zeolite coated Inter-Digitated Capacitor (IDC) element of the sensor, which causes a parasitic shift of the impedance measurements. The impedance of the IDC element is typically measured with a 2000Hz sinewave signal. Typically, the circuits designed to measure the impedance of the gas-sensing cell in this sensor have been designed to minimize any DC leakage currents that would result in a charge on the Zeolite element.
Experimental data has shown that in spite of the efforts to minimize DC leakage currents, sizable amounts of nearly DC voltage can be built-up across the IDC element, due to inter-element leakage with the other sensor component cells. For example, a resistive temperature device (RTD) element that is placed in the sensor to provide feedback for the sensor temperature control function provides a well-known coupling mechanism. In general, the RTD element has been placed in the multi-layer sensor substrate structure, directly under the IDC element level, to achieve accurate temperature monitoring. The historical approach for controlling the effect of this parasitic coupling is to establish a known, fixed potential between the RTD and IDC elements. This potential has been empirically determined and locked into the interface electronics calibration. One issue with this technique is lack of repeatability of the “optimum” bias, due to dependence on sensor age, temperature and several lesser-known influences.
What is needed is a technique for reducing a parasitic DC bias voltage on a sensor, subject to ion migration, that acts to minimize the parasitic DC bias voltage on the sensor over the lifetime of the sensor.